Commonly owned, co-pending U.S. patent applications, Ser. No. 09/344,094, entitled xe2x80x9cFluid Parameter Measurement in Pipes Using Acoustic Pressuresxe2x80x9d, filed contemporaneously herewith, contains subject matter related to that disclosed herein.
This invention relates to the measurement of acoustic waves and more particularly to measurement of acoustic waves in pipes.
It is known that the speed of sound amix of fluids in pipes may be used to determine various parameters of the fluid. It is also known to use ultrasonic acoustic signals as the sound signal measured, to determine the speed of sound. Ultrasonic signals are high frequency, short wavelength signals (i.e., wavelengths that are short compared to the diameter of the pipe). Typical ultrasonic devices operate near 200 kHz, which corresponds to a wavelength of about 0.3 inches in water. Some examples of ultrasonic meters are described in U.S. Pat. No. 4,080,837, entitled xe2x80x9cSonic Measurement of Flow Rate and Water Content of Oil-Water Streamsxe2x80x9d, to Alexander et al., U.S. Pat. No. 5,115,670, entitled xe2x80x9cMeasurement of Fluid Properties of Two-Phase Fluids Using an Ultrasonic Meterxe2x80x9d, to Shen, and U.S. Pat. No. 4,114,439, entitled xe2x80x9cApparatus for Ultrasonically Measuring Physical Parameters of Flowing Mediaxe2x80x9d, to Fick.
The advantage of using wavelengths that are short compared to the diameter of the pipe, is that the fluid behavior approaches that of a fluid in an unbounded media. In an unbounded, homogeneous multi-component mixture, the speed of sound can be expressed as a purely a function of the properties of the components and volumetric phase fractions. In that case, the sound speed is also wavelength (or frequency) independent.
However, as longer wavelengths are used (below the ultrasonic range), the acoustic behavior at the fluid begins to interact with the pipe and surrounding media. The influence of these the boundary effects can fundamentally alter the propagation of sound within the fluid. These effects tend to be wavelength dependant. The propagation velocity (or sound speed) in a bounded system becomes the property of the fluids and the rest of the system with which the fluid interacts. The boundary effects manifest themselves as uncertainty in both measuring and interpreting the sound speed in terms of fluid properties. This uncertainty introduced by the rest of the system diminishes the ability to interpret sound speed measurements. For example, in an oil well, a inner production tube is acoustically coupled to the entire formation and such coupling is dependent on the properties of the produced fluid, the production tubing, the annulus fluid, the casing and the formation.
Furthermore, the associated boundary effects of such coupling introduce dispersion to the propagation of sound within the produced fluid, thereby making the propagation velocity wavelength (and frequency) dependent. This effect is increased as the compliance of the pipe increases.
Objects of the present invention include provision of a system for measuring the acoustic waves in pipes which is not significantly affected by external system characteristics.
According to the present invention, a pipe having at least two acoustic sensors that sense acoustic pressures of a produced fluid in a pipe along a sensing section, comprises: an outer sleeve, attached to the pipe at two attachment locations along the pipe, said sleeve forming a cavity between said sleeve and said pipe in the sensing section.
According further to the present invention, said cavity is filled with a cavity material having an acoustic impedance (xcfx81c2) that is much less than the acoustic impedance (xcfx81c1) of the produced fluid.
According still further to the present invention, the produced fluid is a liquid and said cavity is filled with a gas. According still further to the present invention, the produced fluid is a liquid at a liquid pressure and said cavity is filled with a gas at a gas pressure lower than the liquid pressure of the produced fluid. Still further according to the present invention, said cavity is filled with air and the produced fluid comprises oil. Still further according to the present invention, said cavity is evacuated. According further to the present invention, the pressure sensors measure strain on the pipe.
The present invention provides a significant improvement over the prior art by providing a low impedance acoustic media that isolates the acoustics of the produced fluid and the production tube from the surrounding environment. The present invention enables the measurement of propagating long wavelength (with respect to the pipe diameter) acoustic waves such that it can be interpreted with respect to an infinite domain propagation. Also, it is not critical to the present invention how the speed of sound measurement is made, and the invention may be used with active acoustic sources or passive listening techniques. The present invention provides acoustic isolation that enables well defined propagation characteristics and provides advantages associated with isolating the test section from noise from the environment thereby preventing such noise from influencing the measurement of acoustic properties of the fluid in the production tube in the sensing region. The present invention is applicable to many applications including the oil industry, refining, pipe lines, water industry, nuclear industry, or any other application where the speed of sound of a fluid in a pipe or conduit is desired to be measured.
The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof.